Conor McGregor knocked out Eddie Alvarez in the second round to win the UFC lightweight title and become the first two-class champion in the promotion's history.

McGregor dominated from the opening bell of the main event of UFC 205 at Madison Square Garden on Saturday night and the sold-out crowd roaring with each blow. McGregor is now the reigning featherweight and lightweight champ.

UFC has not decided if McGregor will be allowed to defend both championships. UFC President Dana White said McGregor could be about the only fighter in the promotion to handle that kind of fight load.

McGregor crouched inside the cage waiting for the bell to ring and attack Alvarez. McGregor was the clear aggressor from the start and dropped Alvarez three times in the first round. Alvarez, out of Philadelphia, bounced up the first two times and took a severe beating on the third. McGregor forced Alvarez to fight with his back to the cage and dominated the rest of the round.

"This is the biggest event in the history of MMA," UFC color commentator Joe Rogan told the crowd.

UFC stacked the card with three title fights that were expected to help set a gate record of more than $17 million at MSG. The 1999 boxing match between Lennox Lewis and Evander Holyfield drew a record $13.5 million.

UFC had never run a show in New York City because of a two-decade ban imposed by New York that left only unsanctioned and unsafe MMA fights in the state. State lawmakers and Democratic Gov. Andrew Cuomo agreed in April to end the ban following years of failed efforts by MMA supporters. The law authorizing the sport took effect in September.

MSG was packed with nearly 20,000 fans and UFC was on pace to set a gate record for the arena. McGregor, as he had been all week in New York, was the star of the show — even with A-listers Madonna and Hugh Jackman in the arena. McGregor's fans wore Irish flag capes and his fellow countrymen sang "Ole Ole Ole" in the concourse before the show.

UFC last ran a major show in the state at UFC 7: The Brawl in Buffalo on April 7, 1995. UFC, under Lorenzo and Frank Fertitta, exploded into a global phenomenon, become a staple on network television and ran PPV cards that hit 1 million buys during the ban. UFC 205 was expected to reach around 1.5 million PPV buys. UFC sold for approximately $4 billion to a group led by Hollywood entertainment conglomerate WME-IMG in July.

Yoel Romero caught Chris Weidman with a flying right knee and finished him off in the third round with a spectacular, bloody knockout victory. Weidman sat loopy in the octagon as blood poured down his face and onto his chest. Romero sprinted toward the cage, flipped over the top and ran a lap in celebration.

Romero's win made him the No. 1 contender for Michael Bisping's middleweight title and they wasted no time hyping that potential matchup. Bisping was shown on the big screen and extended his middle fingers at Romero.

"It is an honor to be a part of UFC 205 in Madison Square Garden, but now I want my shot at the title," Romero said.

Miesha Tate is out of title shots.

Tate, who played a pivotal role in the women's division rise to prominence in UFC, announced her retirement inside the octagon following a loss to Raquel Pennington. The 30-year-old Tate says the loss played a role in her decision.

"I had a lot more to give but I couldn't pull it out of myself," she said.

Tate defeated Holly Holm in March to win the bantamweight title and then lost the belt in her first title defense to Amanda Nunes in July.

Tate (18-7) had coached Pennington on "The Ultimate Fighter," a reality show used by the UFC to recruit new talent.

The Madison Square Garden crowd gave Tate a loud ovation following her surprise announcement.